One of two female birds released last year, named Beaky, returned to its release site and was seen being “aggressive and domineering” towards three young males released this month. “She immediately began treating the release area as her territory, which is exceptional for such a young bird,” said an SNH statement.

It appears the bird caused the death of one young male with another missing nearby.

The third young male has been safely secured in the aviary at the release area.

Project director Prof Des Thompson of SNH said: “We have never heard of such incidents before, despite having worked with eagle release projects in other parts of Europe. “For the project team, and the estates which kindly allowed us to take these male eagle chicks, this is very difficult. “Our project has been 11 years in the making because it is crucial to ensure everything possible is done to protect the health and wellbeing of these birds.” He said they remained determined for the project to succeed and would share their information with others. Duncan Orr-Ewing, head of species and land management at RSPB Scotland, said: “This is an exceptional event and, while natural, incredibly upsetting. “As the project is carefully monitored this event has been witnessed at first hand. “The project will continue despite this setback, however we will thoroughly consider any future mitigation.”